This invention pertains to mapping electron microscopes and related electron-optical systems with which it is possible to view a specimen surface in two dimensions.
A scanning electron microscope (SEM) generally is used for examining the surface of a specimen, such as the product of a step in a process for manufacturing semiconductor integrated circuits, especially to ascertain the presence of surficial defects. In view of the fact that an electron beam is an exemplary charged particle beam, investigations have been made into the use of other charged particle beams (such as a focused ion beam) for similar applications.
Since principles generally applicable to an electron beam are applicable to an ion beam, the discussion below is made in the context of an electron-beam system. However, in view of the above, it will be understood that the invention is not limited to electron-beam systems.
In an SEM, as is known generally, an electron beam is irradiated onto a point on the surface of the specimen being observed. Impingement of the electron beam on the specimen surface causes the surface to emit secondary electrons. The secondary electrons are accelerated away from the surface, collected, and quantified by a suitable detector. To image a region on the sample, the electron beam simply is scanned in two dimensions in a raster manner. Secondary electrons generated at each irradiation point in the scan are collected and quantified. The data collected by the detector are processed to form an image that is displayed on a screen (CRT) or the like.
A main disadvantage of conventional SEMs is the long period of time required for obtaining an image of the surface being observed. The time is related to the need to scan a point-focused electron beam two-dimensionally over the observed surface. As a result, xe2x80x9cmapping electron microscopesxe2x80x9d are being investigated for use, as a possible alternative to SEMs, in examining semiconductor wafers and chips and in other applications in which high speed is required. This is because a mapping electron microscope offers prospects of simultaneously viewing an entire region of the target surface in two dimensions. To such end, a mapping electron microscope utilizes an electron-optical system (i.e., a system comprising a 2-dimensional projection-electron lens) to direct the electron beam onto an area of the sample surface that is larger than a point. Unfortunately, various technical problems remain unresolved with mapping electron microscopes.
An important technical problem concerns electrostatic charging of the specimen surface that is being observed. Charging can occur whenever the specimen has an insulator or floating conductor. During charging, the irradiated area acquires a positive or negative electrostatic charge whenever the number (quantity) of the electrons irradiating the specimen is not equal to the number (quantity) of electrons emitted from the irradiated surface as secondary electrons and the like. Whenever charging occurs, the observed surface of the specimen is not in a desired equipotential condition; in fact, the localized potentials within the observed field can differ to such an extent (due to localized accumulations of electrostatic charges) that imaging of certain regions is impossible.
In a mapping electron microscope, low-energy electrons, especially secondary electrons and the like, are accelerated and magnified to high magnification and projected by an electrostatic lens onto an imaging surface (e.g., the surface of a detector). The energy band of such electrons that can be imaged is narrow due to defocusing (on-axis chromatic aberrations). Also, energy uniformity across the entire imaging field is difficult to sustain. Serious problems can arise if the distribution of electrical potential varies greatly over the specimen surface because the image in the vicinity of such variations is distorted or cannot be formed at all, making accurate observation impossible. In addition, the specimen itself may be damaged if it becomes charged sufficiently greatly to cause an electrostatic discharge or insulation breakdown.
The occurrence of charging is determined at least in part by the xe2x80x9csecondary-electron production efficiency.xe2x80x9d The secondary-electron production (SEP) efficiency is the current of produced secondary electrons divided by the beam current of charged particles in the beam irradiating the specimen. If the SEP efficiency is greater than unity (1), then the specimen acquires a positive electrostatic charge; if the SEP efficiency is less than unity, then the specimen acquires a negative electrostatic charge. Hence, to avoid the problems summarized above, it would be advantageous if specimen irradiation could be performed (especially with respect to insulators and floating conductors) in a manner by which the SEP efficiency is maintained as close to unity as possible.
However, a typical specimen (especially a patterned semiconductor wafer or chip) typically includes multiple types of insulators and floating conductors each having a different respective SEP efficiency. With such specimens, it is conventionally extremely difficult to observe the specimen by mapping electron microscopy without causing, unacceptable levels of localized charging. Many specimens simply cannot be imaged at all without intentionally charging them at least to a certain extent (e.g., to obtain a potential-contrast image). In such instances, it is difficult or impossible to control the extent of localized or general charging of the specimen.
The shortcomings of the prior art as summarized above are solved by mapping electron microscopes according to the present invention in which the degree of localized charging of the specimen is controlled, especially with respect to insulators and floating conductors. Hence, the charging is maintained between a minimum needed for producing a viewable image and a maximum beyond which a viewable image is not obtainable with sufficiently low distortion or without damaging the specimen.
This invention provides, inter alia, mapping electron microscopes comprising an irradiation-optical system that irradiates the observed surface of a specimen with electrons from an electron source. The mapping electron microscopes also comprise an electron-imaging optical system that collects imaging electrons-emitted from the irradiated surface of the specimen and directs them onto an image-pickup surface (xe2x80x9cdetectorxe2x80x9d).
One embodiment of such a mapping electron microscope comprises an irradiation-optical system, an imaging-electron detector, and a projection-optical system. The irradiation-optical system is situated and configured to irradiate a surface of a specimen with charged particles produced by a charged-particle source, so as to cause the specimen surface to emit imaging electrons. The imaging-electron detector comprises a detection surface. The projection-optical system is situated and configured to direct the imaging electrons in an image-forming way onto the detection surface. The irradiation-optical system is further configured to produce the beam controllably having a characteristic such that localized changes in potential due to charging by the beam at one or more regions of the specimen surface, when irradiated by the beam from the irradiation optical system, are within respective predetermined ranges in which an image of each such respective region can be obtained. The imaging optical system can be further configured such that each of multiple regions on the specimen surface is irradiated so as to acquire a respective change of surface potential (Us) that is greater than a respective minimum change of surface potential (Umin) needed to produce a viewable image and a respective maximum change of surface potential (Umax) beyond which a viewable image cannot be obtained. The mapping electron microscope can include a Wien filter situated and configured to direct the beam of charged particles from the irradiation-optical system to the specimen surface.
Using a Wien filter allows for perpendicular irradiation of the specimen, which facilitates uniform irradiation of the specimen compared to angled irradiation. Koehler illumination conditions can be created by placing an aperture between the Wien filter and the specimen, and aligning the focal point of a cathode lens (located between the aperture and the specimen) with the aperture position.
In another embodiment of a mapping electron microscope according to the invention, the irradiation-optical system is situated and configured to irradiate a surface of a specimen simultaneously with electrons produced by multiple electron sources. The electrons from each source have a respective current and respective incident energy that are controlled independently at each source. The electrons incident on the specimen surface cause the specimen surface to emit imaging electrons. The mapping electron microscope also includes an imaging-electron detector and a projection-optical system that is situated and configured to route the imaging electrons in an image-forming way to the detection surface of the imaging detector. In this embodiment, the irradiation-optical system can comprise the multiple electron sources.
For example, the irradiation-optical system can comprise a separate respective irradiation column corresponding to each electron source. In such a configuration, each electron source produces a respective irradiation beam. The irradiation-optical system can include a Wien filter. The Wien filter is situated and configured to receive each irradiation beam from a respective deflection angle relative to an optical axis, and to direct each respective irradiation beam along the optical axis to the specimen surface. As another example, the irradiation-optical system can comprise a first and a second electron source. The first electron source produces a first irradiation beam at an angle xcex81 relative to the optical axis, and the second electron source produces a second irradiation beam at an angle xcex82 relative to the optical axis. The respective angles xcex81, xcex82 are established according to:
L=(sin xcex81/eB)(2m)xc2xdV11/[(V11)xc2xd+(Vret)xc2xd]
L=(sin xcex82/eB)(2m)xc2xdV12/[(V12)xc2xd+(Vret)xc2xd]
wherein B is a magnetic field produced by the Wien filter fulfilling a Wien condition with respect to secondary electrons accelerated from the specimen surface at a retarding voltage (Vret) formed by the surface potential, e is the absolute value of the electron charge, m is the mass of an electron, V11=V1+Vret wherein V1 is an incident energy of the first irradiation beam, and V12=V2+Vret wherein V2 is an incident energy of the second irradiation beam. Furthermore, the angles xcex81, xcex81, xcex82 can satisfy the following:
sin xcex81/sin xcex82=V12[(V11)xc2xd+(Vret)xc2xd]/{V11[(V12){fraction (1/2 )}+(Vret)xc2xd]}
Further by way of example, each electron source can be configured to produce a respective irradiation beam, and the irradiation-optical system can comprise a respective Wien filter for each irradiation beam. In this configuration, each Wien filter is situated and configured to receive the respective irradiation beam from a respective angle relative to an optical axis, and to direct the respective irradiation beam along the optical axis to the specimen surface. With this embodiment, the multiple electron sources facilitate keeping changes in surface potential of the specimen due to charging of various insulator bodies or floating conductors within respective target values.
In another embodiment of a mapping electron microscope according to the invention, an electron source is configured to produce a beam of irradiation electrons. The beam has a first segment containing irradiation electrons at a first beam current and incident energy, and a second segment containing irradiation electrons at a second beam current and incident energy. The second segment is in temporal series with the first segment. The embodiment also includes an irradiation-optical system situated and configured to irradiate a surface of a specimen with the beam of irradiation electrons produced by the electron source. The irradiation-optical system is further configured to separate the second segment temporally from the first segment and to control the beam current and incident energy of at least one segment as incident on the specimen surface. The embodiment also includes an imaging-electron detector comprising a detection surface, and a projection-optical system. The projection-optical system is situated and configured to route the imaging electrons in an image-forming way to the detection surface of the imaging detector. This mapping electron microscope can include a Wien filter situated and configured to direct a beam of charged particles from the irradiation-optical system to the specimen surface.
Hence, in this embodiment, specimen irradiation is performed by dividing the current and incident energy from at least one electron source into temporal (time) segments. The achieved result is the same as if there were multiple electron sources each producing a respective irradiation beam having a respective beam current and incident energy. This is because charging of locations on the specimen surface is chronologically and spatially overlapping.
The detector can include an imaging-electron converter and a photoelectric converter, such as a CCD. The detector receives the imaging electrons and converts them to a corresponding electrical signal. The charge per each temporal cycle of irradiation is accumulated by the CCD. Respective outputs from the CCD for the different irradiation cycles are output to produce an image.
Desirably, irradiation is performed under uniform irradiation conditions within the irradiation field. This produces a clear image without lightening or darkening of the image based on localized charging or irradiation irregularities within the field.